The below-illustrated sodium (2S,3S)-3-[[(1S)-1-isobutoxymethyl-3-methylbutyl]carbamoyl]-oxirane-2-carboxylate (hereinafter referred to as Compound A) shows a cathepsin-inhibitory action and is useful as a remedy for treating rheumatoid arthritis and osteoporosis (Patent reference 1: WO 99/11640 pamphlet):

Patent reference 1 describes a process for producing Compound A according to the following reaction scheme (see Example 48 of Patent reference 1):

The starting ester material of the above-mentioned reaction scheme is generally purified by column chromatography to obtain a purified Compound A.